Tissue paper is an absorbent paper which exhibits a specific surface weight preferably between 15 and 35 g/m2 and which can be manufactured to be stretchable. A present-day technique applies and glues a still moist sheet while on a drying drum and then detaches it off this drum using a scraper blade in order to produce creping corrugations. The sheet then can be wound on a reel to await transformation into the finished product. This summarized technique is called conventional. It is denoted as CWP in the field.
Another technique consists in drying the sheet after it was drained but without applying pressure to it, at least in part until the dryness suffices to fix in place the fibers inside the sheet. Where appropriate, drying is finalized by placing the sheet on a heated cylinder. Thanks to this first drying stage, the sheet may be pressed against the cylinder without degrading the sheet's structure. The sheet retains part of its volume. This cylinder furthermore allows creping. The first drying action is carried out in the absence of excess pressure by blowing hot air through the sheet after it was drained. This technique is denoted as TAD in the field and allows producing thicker sheets of higher density than the conventional technique. It is characterized by a more open structure and greater permeability.
Thereupon and by means of the embossing technique, the sheet's properties may be improved or be at least modified, for instance softness, flexibility, absorption, thickness or appearance. Final finishing then depends on the final use. The embossing pattern typically is in the form of protrusions or bosses of pyramids of circular, oval or square cross-sections which are regularly spread over the surface of the sheet. Be it noted that a protrusion on one side of the sheet subtends a cavity or alveole on the other.
In order to emboss tissue paper, a rigid cylinder is generally used at the surface of which rise embossing tips produced by engraving or machining and which assume appropriate shapes, sizes and densities. The sheet is placed against the cylinder and then compressed by another cylinder fitted with a deformable cladding, for instance rubber. In this manner, the clad cylinder hugs the protrusions of the other cylinder. For a given pattern, the ensuing deformation depends on the selected parameters such as the cladding rubber's resiliency, its deformability and its ability to hug the engraving topography, and the embossing pressure.
At present applicant markets a paper towel of which the protrusions are arrayed in concentric circles. This paper towel consists of two plies of tissue paper that were embossed separately and which were assembled so that the protrusions are in so-called mutually “nested” positions. The protrusions of one ply are opposite the protrusions of the other ply and configured between each other. In this manner the two plies nest in each other and subtend air pockets and thus improve absorption. The sheet includes alveoles at its two visible sides. The appearance of such a sheet, not under the microscope but to the naked eye, exhibits a two-tier topography. A first tier is defined by the bottom of the alveoles (this bottom being in a single plane provided that the embossing tips are all at the same level). The second tier corresponds to the unembossed sheet portion.
In order to improve a paper towel's absorptivity, it can be processed at the time of its manufacture, that is on the papermaking machine, with respect, for example, to its constitution, its drying mode or any additives.
In this respect the sheet's embossing mode also is a significant parameter.